We have developed a diode-laser-based Frequency Domain Photon Migration (FDPM) device optimized for clinical optical property studies to characterize breast tissue in vivo . FDPM utilizes intensity-modulated, near-infrared (NIR) light to measure tissue absorption and scattering properties. The wavelength-dependence of absorption is used to determine tissue hemoglobin concentration (total, oxy- and deoxy- forms), oxygen saturation and water concentration. Our goal is to move towards developing a simple, rapid, hand-held optical screening device which will yield low resolution tissue functional information that complements and enhances the detailed structural information obtained from x-ray mammography. In the clinical studies abnormal and normal regions of breast tissue in patients with detected breast abnormalities are measured prior to surgery and in vivo optical properties are determined. The goal of clinical FDPM studies is to provide an optical basis for d isti nguishing and characterizing healthy and diseased tissue. These optical properties are correlated with pathology reports and ultrasound images to gain an understanding of the physiological basis of the observed data. An additional study of volunteers with no known breast abnormalities are studied to gain insight in normal optical property variations due to change in menopausal status, menstrual cycle fluctuations, hormone supplements and spatial variation.